Linear birefringence in a single-mode fiber optic sensing coil employing the Faraday effect has the effect of reducing the sensitivity of the coil to magnetic fields. The sensitivity decreases with increasing length and birefringence of the coil. Birefringence occurs even if the sensing fiber is straight, due to the presence of residual linear birefringence resulting from core ellipticity. Even nominally circular core single mode fibers possess some core ellipticity due to imperfections in the production process. Additionally, bending or other anisotropic stresses on the core contribute to birefringence, such as when the fiber is formed into a coil.
Linear birefringence tends to suppress the response of the fiber to magnetic fields. As this effect is temperature dependent, and each fiber has different characteristics, it is difficult to produce a sensing coil whose characteristics are known over the operating temperature range. Moreover, measuring the coil temperature may be impracticable and expensive.
In the short section of fiber that is used for a sensing coil (a few meters to 50 meters), the ellipticity is relatively constant. This arises from the method of manufacture of optical fiber, where the fiber is drawn down from a preform. A preform is a version of the fiber that has a larger diameter than the fiber, but is shorter in length. Exemplary length ratios (fiber/preform, for fibers manufactured by KVH Industries) are approximately 4000:1. In other words, 0.25 mm of preform length yield approximately 1 meter of fiber. Over this short preform length the ellipticity will not change significantly, so that the ellipticity can be considered as being constant in that section of fiber.
Several approaches have been proposed to overcome the deleterious effects caused by linear birefringence when the fiber is used for current sensing. For example, circular birefringence can be introduced by torsion-type twists to “swamp” the linear birefringence and achieve coupling of the linear modes. Alternatively or in addition, the preform or the fiber can be spun or rotated in an oscillating manner during drawing to reduce polarization mode dispersion (PMD). These remedies tend to complicate the fiber or coil fabrication process, may require annealing, and are expensive.
It would therefore be desirable to provide an optical fiber for magnetic field and current sensing applications and a method of producing such an optical fiber that has increased sensitivity to a magnetic field and reduced temperature dependence by reducing the deleterious effects caused by linear birefringence and that can be produced at less cost.